1. Field of the Invention
This invention relates to a method and apparatus for reproducing information signals from an optical disc by radiating the light to the disc. More particularly, it relates to such method and apparatus adapted for compensating for deterioration in playback signals produced by tilt in the optical disc.
2. Description of Related Art
An optical disc is a recording medium having an extremely wide field of application as a package medium for information signals. On the optical disc, there are recorded data strings modulated in accordance with a pre-set modulation system, such as eight-to-fourteen modulation (EFM).
FIG. 1 shows a portion of a replay-only optical disc 61.
On the replay-only optical disc 61, there are recorded data strings extending along a spiral information track 62 in the form of pits 64. For reproducing the data strings recorded on such optical disc 61 as information signals, a light beam is radiated from a light source, preferably a laser diode (semiconductor laser), and changes in the volume of the reflected light from the disc are detected. Specifically, when the light beam is radiated on the information track 62, the light is reflected from a mirror portion 63 free of pits 64 in a larger quantity, while it is reflected from the pits 64 in a lesser quantity. In an optical disc reproducing apparatus, such as a compact disc player or a video disc player, the information signals are reproduced by taking advantage of such change in the light quantity.
The optical disc reproducing apparatus includes an optical pickup having a laser diode as the light source, a photodetector for detecting the quantity of the reflected light, and an optical system made up of a collimator lens, an objective lens and a beam splitter for collecting the light beam from the laser diode on the information track 82 on the optical disc 81 and for routing the reflected light from the optical disc 81 to the photodetector. The frequency characteristics of the optical pickup can be defined in terms of the spatial frequency. The frequency characteristics are generally represented using a function known as a modulation transfer function (MTF). The optical cut-off frequency fc of the MTF is primarily represented by an equation EQU fc=2NA/.lambda. (1)
wherein .lambda. denotes the wavelength of the light of the light source and NA the numerical aperture of the objective lens employed in the optical pickup.
If the pits 64 and the mirrors 63 are alternately recorded at a frequency not less than the cut-off frequency fc shown in the equation (1), it becomes wholly impossible to read out information signals.
In addition, the MTF gain characteristics are not flat up to the cut-off frequency fc, but are decreased monotonously. The result is the band-limited playback waveform.
Since the optical disc is a recording medium, it is convenient if more information signals can be recorded on the same size disc. However, since there is a limitation imposed by MTF on the spatial frequency that can be read optically, it is not that easy to raise the recording density for the same system.
For realizing the higher recording density, either the numerical aperture NA of the optical lens of the optical pickup is increased, or the wavelength .lambda. of the light of the light source is diminished. It is technically most difficult to reduce the wavelength .lambda. of the light of the light source in view of the necessity of keeping the small size of the laser unit. Thus the technique of enlarging the numerical aperture NA of the objective lens for increasing the recording density of the optical disc is under investigation.
Judging from the cut-off characteristics of the MTF, it may be contemplated that the larger the numerical aperture NA of the objective lens, the smaller becomes the beam spot of the light beam, thus possibly leading to improved resolution and higher recording density. However, although the higher recording density may be achieved by increasing the numerical aperture NA, the apparatus is deteriorated in operational stability. By far the most inconvenient is the fact that the allowance for skew which is the disc tilt is drastically lowered by enlarging the numerical aperture NA. The optical disc surface cannot be machined to a geometrically satisfactory planar surface, while the disc may be warped in the course of manufacture. On the other hand, an optical disc cannot necessarily be set in a ideally horizontal position when mounted on the optical disc reproducing apparatus. Consequently, disc skew cannot be removed completely and hence the numerical aperture cannot be increased without restrictions.
Specifically, if the optical disc is tilted relative to the optical axis of the objective lens, the coma aberration is generated in proportion to approximately the third power of the numerical aperture NA and to approximately the first power of the quantity of skew .THETA.. If represented by, for example, the Seitel's aberration coefficient formula, the coma aberration is approximately EQU t.multidot.(n.sup.2 -1)/2n.sup.3 .multidot.NA.sup.3 ( 2)
for a sufficiently small skew quantity .THETA..
In the above formula, t denotes the thickness of an optical disc substrate and n the refractive index of the optical disc substrate. If, for example, an objective lens with a numerical aperture of 0.6, which is 1.33 times as large as the numerical aperture of 0.45 of the objective lens of the optical pickup of, for example, a compact disc player, is employed, the coma aberration 2.37 times as large as that of the compact disc is generated despite the fact that the quantity of skew is on the same order of magnitude as that of the compact disc. Due to tilt of the reflection surface, that is wave front distortion, the light spot formed on the optical disc becomes non-symmetrical, such that it becomes difficult to extract the signal sufficiently.
Thus it has been envisaged to detect the skew and to make adaptive skew correction by a skew correcting device depending on the detection signal.
First, a skew sensor for detecting the skew is explained by referring to FIG. 2.
The skew sensor is made up of a light emitting diode (LED) 71, a two-segment photodetector 72 and a lens 73. The lens 73 may be resin-molded as-one with the LED 71 and the two-segment photodetector 72. The light beam radiated from the LED 71 on the optical disc 61 is reflected thereby to form a light spot 74 on the two-segment photodetector 72.
If the optical disc 61 is tilted, the light spot 74 is moved on the two-segment photodetector 72 in a direction of segment separation as indicated by an arrow LR in FIG. 2. The differential output of the two-segment photodetector 72 is produced by an additive node 75. The differential output is supplied as a skew error signal to the skew correction device.
An output signal of the skew sensor shown in FIG. 2 is shown in FIG. 3. The skew sensor shown in FIG. 2 utilizes the linear range of the output signal waveform shown in FIG. 3.
The skew correction device utilizes the output signal of the above skew sensor for adaptively controlling the optical disc skew. As such skew correction device, there is known a device employing two correction plates.
The skew correction device drives the two correcting pates to an optimum state, using the skew error signal outputted by the skew sensor, in order to render the optical disc reproducing apparatus strong against skew up to a certain quantity of skew. This it is possible with the optical disc reproducing apparatus to enlarge the numerical aperture NA of the objective lens to some extent.
Meanwhile, for reproducing the optical disc using the optical disc reproducing apparatus, as described above, the skew sensor is employed for producing the information as to the degree of tilt of the optical disc. However, the skew sensor is not high in accuracy, while an error may be produced in the skew sensor offset or an error may be produced due to changes in temperature. Unless the skew sensor is improved significantly in mounting accuracy, the measurement error tends to be produced since the optical axis of the light from the light source is coincident on only rare occasions with the optical axis of the LED of the skew sensor. Due to these factors, it is likely that the correct degree of tilt cannot be obtained with the skew sensor.
For obtaining the tilt of the disc portion illuminated with the beam spot, the skew sensor needs to be mounted on the opposite side of the optical disc with respect to the optical pickup. However, in such case, the optical disc reproducing apparatus is increased in size contrary to the demand for reduction in size and weight of the device.
In addition, with an optical disc reproducing apparatus for reproducing information signals by introducing an optical disc cartridge comprising an optical disc contained in a main cartridge body, or with an optical disc reproducing apparatus aimed at size reduction, the skew sensor is necessarily disposed on the same side of the optical disc with respect to the optical pickup. In such case, it is possible to obtain the tilt of the surface in the vicinity of the disc portion irradiated with the beam spot, while it is not possible to obtain the tilt of the disc portion itself. Consequently, should there be any distortion in the disc surface itself, the disc portion irradiated with the beam spot and the disc portion measured by the skew sensor become different in tilt, such that it may occur that correct tilt magnitude cannot be recognized.